1. Field of the Invention
This invention relates to silicone elastomeric foam produced by expanding an aqueous emulsion, then curing by removing the water with microwave energy.
2. Description of the Prior Art
A general method of foam production from elastomeric emulsions is known as the Talalay process. As described in U.S. Pat. No. 2,432,353, issued Dec. 13, 1938, a natural rubber latex is compounded with curing agents, then caused to expand with hydrogen peroxide. The expanding mixture is poured into an aluminum mold, the mold filled with foam is immersed in brine at -30.degree. C. to freeze the contents. The frozen latex is coagulated by drawing alkaline calcium chloride brine at -2.degree. C. through the foam. The mold with the coagulated foam is then exposed to live steam at 125.degree. C. to vulcanize the natural rubber. Talalay teaches the preliminary formation of the foam can be effected by any of the known procedures, such as mechanical whipping, chemical gas generation, or physical release of gas or vapor.
In the book "Neoprene Latex", by J. C. Carl, published by E. I. Dupont DeNemours and Co. (Inc.) (1962) a process of converting specialized neoprene latex to foam is described. The latex fillers, curing agent, accelerator, and foam stabilizing surface active agents are mixed, then the mixture is stirred rapidly to whip in air and create a froth. The froth is stirred until it is refined to a smooth cream. A gelling agent is then added and the froth placed in a mold. The foam produced by the gelling of the froth is usually cured by exposure to steam. After curing, the foam is washed and dried.
The processes as discussed above require the gelling of the froth to stabilize the foam by the use of either an internal gelling agent as discussed for a neoprene latex or an external gelling agent as discussed with a natural rubber latex. This gelling step must be carefully controlled in order to produce satisfactory foam. Dunn in U.S. Pat. No. 3,491,033, issued Jan. 20, 1970, described a process of making a solid cellular material that does not contain a gelling step. Dunn describes his process as applicable to any polymer that is available in the form of an aqueous emulsion and is film forming at a temperature below 300.degree. F. Dunn shows examples of foam production with natural rubber latex, butadiene-styrene latex, and also emulsions of thermoplastic polymers such as polyethylene and vinyl chloride. His process is carried out by adding a foaming agent, incapable of gelling the wet foam, to an aqueous dispersion, forming the mixture into a wet foam, drying the wet foam into a solid cellular material, and heating the solid cellular material to the film-forming temperature of the polymer. Dunn uses relatively large amounts of foaming agents to allow the foam structure to retain its cellular shape during the drying step. The emulsion used in the instant invention does not fall under Dunn, as the emulsion cannot be dried to a solid cellular material as called out by Dunn without gelling the foam, which is contrary to the requirement of Dunn.
Kittle and Ronk in U.S. Pat. No. 4,026,844, issued May 31, 1977, teach foaming a composition of polydiorganosiloxane gum base stock, hydroxyl containing organosiloxane, organohydrogensiloxane, acetylenic alcohol inhibitor, platinum, and a microwave sensible material. The mixture is foamed and cured by exposing the composition to a microwave source. The microwave energy heats the microwave sensible material, which in turn heats the other ingredients, resulting in a cured foam.